23/64 field points are widely available, and fit a 3/8 shaft just fine. And you can taper any diameter shaft to fit a 5/16 point if you want, it might look a little small on a 3/8 shaft, but it will be functional and all. (Ok, a 5/16 point might look a little more than a "little" small on a 3/8"shaft, but you can't really tell the difference between a 23/64 on a 23/64 shaft from a 23/64 on a 3/8 shaft, as least I can't.)